Electroacoustic transducer

ABSTRACT

An electroacoustic transducer having a pair of spaced confronting permanent magnet plates and a vibratable diaphragm disposed between the magnet plates and carrying a conductor. The magnet plates have circular concentric poles on their confronting faces with poles of like polarity registering with each other across the diaphragm. The conductor on the diaphragm follows a spiral path and each zone of the diaphragm confronting an interpolar transitional surface region of the magnetized plate faces is provided with a plurality of spiral convolutions of the conductor. The spiral convolutions of adjacent zones have opposite directions.

United States Patent 11 1 Beer 1 1 ELECTROACOUSTIC TRANSDUCER [22]Filed: June 14. 1973 1211 Appl. No.1 369,825

[30] Foreign Application Priority Data June 20. 1972 Sweden 8109/72 [52][1.5. CI 179/1155 PV: 179/1155 H [51] int. Cl. H04R 9/02 [58] Field ofSearch 179/1155 PV. 115.5 R.

[56] References Cited UNITED STATES PATENTS 3.013.905 12/1961 Gamzon et179/1155 R 3.139.491) 6/1964 L \ons 179/1155 PV 3.2091184 9/1965 Gamzonet a1. 179/1155 P\" 3.283.086 11/1966 Evans 262.911) 12/1926 UnitedKingdom 1 1 Nov. 11, 1975 1.329.295 7/1962 France 179/115 5 484.339111/1929 German} 179/1155 P\' 1.430.700 4/1965 France 179/1155 H PrimaryE.\'aminerl(athleen H. Claffy Assistant Erumz'ner-George G. StellarAttorney. Agent. or Firm-Hill. Gross. Simpson. Van Santen. Steadman.Chiara & Simpson [57] ABSTRACT An electroacoustic transducer having apair of spaced confronting permanent magnet plates and a vibratablediaphragm disposed between the magnet plates and carrying a conductor.The magnet plates have circular concentric poles on their confrontingfaces with poles of like polarity registering with each other across thediaphragm. The conductor on the diaphragm follows a spiral path and eachzone of the diaphragm confront ing an interpolar transitional surfaceregion of the magnetized plate faces is provided with a plurality ofspiral convolutions of the conductor. The spiral convolutions ofadjacent zones have opposite directions.

12 Claims, 7 Drawing Figures U.S. Patent Nov. 11, 1975 FIG. 3

B /4 ZEN Hm w I v N ELECTROACOUSTIC TRANSDUCER BACKGROUND OF THEINVENTION This invention relates to electroacoustic transducers of theelectrodynamic type for use in loudspeakers. earphones or microphones.More particularly. the invention relates to electroacousticelectrodynamic transducers of the type having a vibratable diaphragmwhich carries a conductor for electrical current and is disposed in apermanent magnetic field. Transducers of this type are illustrated anddescribed in US. Pat. Nos. 3,013,905 and 3,l64,686.

An advantage of transducers of this type is that they can easily beconstructed such that the diaphragm vibrates substantially uniformly inphase over its entire movable surface. However. known transducers ofthis type have a construction such that they are relatively expensive tomanufacture and, moreover, the conductor cannot easily be given the highimpedance that is desirable especially in the case of microphones andearphones. The present invention provides an electrodynamicelectroacoustic transducer which avoids these and other drawbacks.

BRIEF SUMMARY OF THE INVENTION In accordance with the invention there isprovided an electrodynamic electroacoustic transducer comprising apermanent magnet having a magnetized face. a vibratable diaphragmconfronting this face in spaced parallel relation therewith, and acontinuous flexible conductor which is carried by the diaphragm. Themagnetized face of the permanent magnet provides a north pole and asouth pole and one of the poles forms a closed loop around the other. Atransitional surface region of the magnetized face separates the twopoles, and the magnetic field produced between the poles thus traversesthis surface region. The diaphragm is disposed in this magnetic fieldand the conductor on the diaphragm is arranged such that the lines ofthe mag netic field intersect it substantially at right angles in thezone of the diaphragm confronting the transitional surface region.Moreover, in that zone of the diaphragm the conductor comprises aplurality of spiral convolution so that each line of force of themagnetic field bridging the transitional surface region intersects theconductor a plurality of times.

In a preferred embodiment the magnet is a plate of a magneticallyanisotropic material which has been magnetically oriented with thedirection of preferred magnetization generally parallel to the faceconfronting the diaphragm, so that the magnetic field exists predominantly at that face and only minimally at the opposite face. In this waya strong magnetic field can be obtained.

A stronger and more uniform magnetic field surrounding the conductor maybe obtained if two similar permanent magnets are used which are disposedon op posite sides ofthe diaphragm with poles oflike polarity inregister with each other across the diaphragm.

The above and other features and advantages will be clear from thefollowing detailed description of exemplary preferred embodimentsillustrated in the appended drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a view in axial section of atransducer ac' cording to the invention intended for use in an earphone;

FIG. 2 is a view on line II-ll of FIG. I;

FIG. 3 is a view in axial section of the magnets of the transducer shownin FIG. 1:

FIG. 4 is a view on line IVIV of FIG. 3;

FIG. 5 is a view in axial section ofa modified embodiment;

FIG. 6 is a plan view, generally corresponding to FIG. 2, of thediaphragm of the embodiment of FIG. 5;

FIG. 7 is a plan view. generally corresponding to FIG. 4, of themagnetized face of one of the magnets of the embodiment of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION In the drawing the dimensions andspacings of the elements are somewhat disproportionate as compared tothe actual dimensions and spacings. Examples of actual dimensions andspacings will be given hereinafter in order that the invention may bebetter understood.

Referring to FIGS. 1 to 4, these figures show an electroacoustictransducer 10 intended to be used in an earphone. The sound generatingelement is a generally flat, circular diaphragm 11 which is clampedalong its peripheral portion between two rings I2 held in place in acircular housing 13 by a retaining clip 14. The diaphragm 11 is madefrom a polyester film ofa thickness of about 0.0l mm and has circularcorrugations I5 concentric with the peripheral portion and thus is veryflexible. To one side of this film there is laminated an aluminum foilof a thickness of about 0.0l mm. From this foil a continuous conductor16 is formed which forms a multiplicity of spiral convolutions defininga current path between peripheral and center portions 17 and 18 of thefoil as described in more detail hereinafter.

A pair of flat, circular permanent magnet plates 20 are mounted in thehousing [3 on opposite sides of. and equally spaced from, the diaphragm11. The confronting faces 21 of the plates are concentric with andparallel to each other and to the diaphragm and define between them anair gap 22 in which the diaphragm can vibrate. The center portion of thediaphragm and the center portion of the plates are clamped together bymeans of a bolt 23 and nuts 24. Spacers 2S and 26 on the bolt andflanges on the rings 12 maintain the diaphragm and the plates in theproper spaced relation.

Each plate 20 is provided with a multiplicity of perforations 27. Thewalls of each perforation form a exponential horn directed away from theface 2] and having its axis parallel to the axis of the plate.

The plates 20 are magnetized is such manner that the confronting faces21 each have three poles, namely a circular north pole at the center. acircular south pole between the periphery and the center and a circularnorth pole along the periphery. Thus, each plate produces two annularconcentric permanent magnetic fields having a common south pole. Themagnetization is shown diagrammatically in FIGS. 3 and 4 where theperforations 27 have been omitted for clarity. From these figures it isseen that the direction of polarization is radial and that the magneticfields exist predominantly at the face 21 and only minimally at theopposite face of each plate. This will be explained in more detail asthe description proceeds.

In FIG. 4 the magnetic poles are represented by concentric annularfields designated N and S and separated by annular fields designated 28,hereinafter referred to as transitional surface regions. It should benoted that this representation is for illustrative purposes only, andthat in actual fact the poles and the transitional surface regions arenot as sharply defined as FIG. 4 indicates but merge gradually.Therefore, the annular fields N and S actually show only the centers ofthe poles, and the annular fields 28 show regions in which the magneticfields are strong. Since both plates are magnetized in exactly the samemanner, the poles and the transitional surface regions of both platesare congruent.

The plates may be formed from any suitable permanent magnet material butpreferably are made from a highly coercive magnetically anisotropicmaterial, such as plastic-bonded barium ferrite or a cobolt rare earthmagnet material (one material of the last-mentioned class is known asGECOR and sold by General Electric Company). In the illustratedembodiment, the plates are made from the first-mentioned material (knownas Ferroxdure), and before the magnetization of the plates the materialhas been oriented such that the plates have a preferred direction ofmagnetization that is radial and generally parallel to the confrontingfaces 21. As a consequence. the magnetic fields exist predominantly atthe confronting plate faces 21 and only minimally at the opposite faces.and in addition they can be made very strong.

As shown in FIG. 2 the conductor 16 forms a continuous current pathbetween the peripheral and central portions 17 and 18. Moreparticularly. the conductor forms a multiplicity of spiral convolutions30 about the central portion 18. In the interest of clarity onlyrelatively few convolutions are shown while in the actual embodiment thenumber of convolutions may be several hundred; thus, the width of theconductor may be of the order of0.l mm and the spacing of theconvolutions may be of the order of 0.01 mm.

The direction of the convolutions is reversed about halfway between theperipheral and the central portion, forming respective conductorsections, so that a current flowing in the conductor reverses directionat a point that is opposite the south poles of the magnet plates, thatis. opposite the place where the magnetic field in the air gap 22changes direction. Thus, the spiral convolutions 30 cover two concentricannular zones of the diaphragm 11, one outer zone confronting the outertransitional surface regions 28 of the magnet plates 20 and one innerzone confronting the inner transitional surface regions.

The conductor 16 is electrically connected to two terminal members, oneformed by a lug 31 secured to the peripheral foil portion 17 and theother formed by the bolt 23 and the spacer 25 which are in electricalcontact with the central foil portion I8. Through these terminals thetransducer can be connected in the output circuit of an audio amplifier.for example.

The general mode of operation of the transducer shown in FIGS. I to 4 issimilar to that of the known transducers of the same type. It should benoted. however, that by virtue of the described arrangement of theconductor in a multiplicity of spiral convolutions, the conductor can bemade very long to have the high impedance that is often desired.Moreover. as a consequence of the threepole magnetization of the magnetplates and the reversal of the direction of the spiral convolutions, theinfluence of disturbing exterior mag- 4 netic fields is eliminated; theconductor acts as a bifilar winding.

In FIGS. 5 to 7 a transducer IA is shown in which the diaphragm 11A hasthe conductor 16A on both sides and is held stationary only at itsperipheral portion [7A. Although no corrugations on the diaphragm areshown, the diaphragm may be provided with circular corrugations.Moreover, the two permanent magnet plates 20A are magnetized such thatthey have four circular and annular poles of alternating polarity. seeFIG. 7. Although no perforations are shown. it will be appreciated thatthe magnet plates preferably are acoustically transparent.

The conductor configuration on one side of the diaphragm is the same atboth sides of the diaphragm 11A and shown in FIG. 6 in which in theinterest of clarity the conductor is shown as a single spiral linebetween the peripheral and central foil portions 17A and 18A. Sincethere are three transitional surface regions 28A on the confrontingfaces of the magnet plates, the sprial convolutions 30A reversedirection twice. The two portions of the conductor on either side of thediaphragm are electrically connected with each other at the center ofthe diaphragm as shown at 32 in FIG. 5. The peripheral foil portions 17Aare electrically connected with a pair of terminal lugs 31A.

Although in the illustrated embodiments the diaphragm and the permanentmagnet plates are circular, these elements may be square, rectangular,or oval or have any other suitable shape. Naturally, the shape of thespiral convolutions must correspond to the shape of the poles and thetransitional surface regions. Moreover, in some applications it may besufficient to have a magnet only on one side of the diaphragm.

While in most cases the best results are obtained if the permanentmagnet plates have a relatively large number of poles, two poles maysometimes give satisfactory performance. In that case that is only onetransitional surface region and all spiral convolutions run in one andthe same direction.

What is claimed is:

1. An electrodynamic electroacoustic transducer comprising:

a. a unitary permanent magnet plate having a plurality of magnetic poleson one face thereof at least two of the poles extending along respectiveclosed loops around a central pole. with said poles alternating inpolarity. and forming respective transitional surface regions separatingadjacent poles of opposite polarity. the opposite face of said platebeing directed to the atmosphere;

b. a flexible diaphragm, of nonmagnetic and electrically nonconductingmaterial, having a peripheral portion held stationary with respect tothe magnet plate and a vibratable portion, disposed in the magneticfields defined by said poles. bridging the tran' sitional surfaceregions between adjacent poles, and in spaced confronting relation withsaid one face of the magnet plate; and

c. a continuous flexible conductor for carrying electrical current. saidconductor being affixed to the vibratable diaphragm portion confrontingthe tram sitional surface regions between said pairs of adjacent poles,said conductor comprising a plurality of concentric sections, eachsection having a plurality of mutually adjacent unidirectional spiralconvolutions, with the convolutions of each section corifronting arespective transitional surface region S and reversed in direction withrespect to the convolutions of any section adjacent thereto.

2. An electrodynamic electroacoustic transducer according to claim 1,wherein said flexible diaphragm also has its central portion heldstationary with respect to the magnet plate; and a pair of fixedterminal members disposed respectively at the central portion and theperipheral portion of said one face of said magnet plate andelectrically connected with respective ends of said conductor.

3. An electrodynamic electroacoustic transducer according to claim 1,wherein said magnetic plate has a central magnetic pole and two annularmagnetic poles concentrically disposed with respect thereto, and saidconductor comprises two sections.

4. An electrodynamic electroacoustic transducer according to claim 1,wherein said magnetic plate has a central magnetic pole and threeannular magnetic poles concentrically disposed with respect thereto, andsaid conductor comprises three sections, the innermost and outermost ofwhich have the convolutions thereof running in the same direction, withthe intermediate section having its convolutions reversed with respectto the other two sections.

5. An electroacoustic transducer as set forth in claim I, including asecond unitary permanent magnet plate having a plurality of magneticpoles on one face thereof with the pattern of such magnetic poles andintrapolar transitional surface regions thereof being congruent with thepattern of magnetic poles and intrapolar transitional surface regions ofthe first-mentioned magnet plate, the two magnet plates being disposedon opposite sides of the diaphragm with the poles and the transitionalsurface regions of one magnet plate facing and registering respectivelywith the poles and the transitional surface regions of the other magnetplate, the oppositc face of said second magnet plate being directed tothe atmosphere.

6. An electroacoustic transducer as set forth in claim 1 in which themagnet plate is formed with a multiplicity of perforations. the walls ofwhich form exponential horns having their axes extending transversely ofsaid one face. each perforation in the magnet plate connecting the airspace between said diaphragm and said magnet plate with the atmosphere.

7. An electroacoustic transducer as set forth in claim 1 in which saidpermanent magnet plate is magnetized predominantly at said one face andonly minimally at the opposite face.

8. An electroacoustic transducer as set forth in claim 7 in which themagnet plate is made from a magneti cally anisotropic material and thushaving a direction of preferred magnetization. its actual magnetismbeing aligned with said direction of preferred magnetization, saidactual magnetism being generally parallel to said one face at thetransitional surface region.

9. An electroacoustic transducer as set forth in claim 1 in which saidone face of the magnet plate is circular and flat and the poles and thetransitional surface region are annular and concentric with said oneflat face.

10. An electroacoustic transducer as set forth in claim 9 in which thevibratable diaphragm portion has circular corrugations concentric withsaid one flat face.

11. An electroacoustic transducer as set forth in claim 9 which includesmeans secured to said magnet plate and holding the center portion of thediaphragm in stationary spaced relation to said magnet plate.

12. An electroacoustic transducer as set forth in claim 11 in which saidholding means also is a fixed terminal member connected to one end ofthe conductor.

1. An electrodynamic electroacoustic transducer comprising: a. a unitarypermanent magnet plate having a plurality of magnetic poles on one facethereof at least two of the poles extending along respective closedloops around a central pole, with said poles alternating in polarity,and forming respective transitional surface regions separating adjacentpoles of opposite polarity, the opposite face of said plate beingdirected to the atmosphere; b. a flexible diaphragm, of nonmagnetic andelectrically nonconducting material, having a peripheral portion heldstationary with respect to the magnet plate and a vibratable portion,disposed in the magnetic fields defined by said poles, bridging thetransitional surface regions between adjacent poles, and in spacedconfronting relation with said one face of the magnet plate; and c. acontinuous flexible conductor for carrying electrical current, saidconductor being affixed to the vibratable diaphragm portion confrontingthe transitional surface regions between said pairs of adjacent poles,said conductor comprising a plurality of concentric sections, eachsection having a plurality of mutually adjacent unidirectional spiralconvolutions, with the convolutions of each section confronting arespective transitional surface region and reversed in direction withrespect to the convolutions of any section adjacent thereto.
 2. Anelectrodynamic electroacoustic transducer according to claim 1, whereinsaid flexible diaphragm also has its central portion held stationarywith respect to the magnet plate; and a pair of fixed terminal membersdisposed respectively at the central portion and the peripheral portionof said one face of said magnet plate and electrically connected withrespective ends of said conductor.
 3. An electrodynamic electroacoustictransducer according to claim 1, wherein said magnetic plate has acentral magnetic pole and two annular magnetic poles concentricallydisposed with respect thereto, and said conductor comprises twosections.
 4. An electrodynamic electroacoustic transducer according toclaim 1, wherein said magnetic plate has a central magnetic pole andthree annular magnetic poles concentrically disposed with respectthereto, and said conductor comprises three sections, the innermost andoutermost of which have the convolutions thereof running in the samedirection, with the intermediate section having its convolutionsreversed with respect to the other two sections.
 5. An electroacoustictransducer as set forth in claim 1, including a second unitary permanentmagnet plate having a pluralIty of magnetic poles on one face thereofwith the pattern of such magnetic poles and intrapolar transitionalsurface regions thereof being congruent with the pattern of magneticpoles and intrapolar transitional surface regions of the first-mentionedmagnet plate, the two magnet plates being disposed on opposite sides ofthe diaphragm with the poles and the transitional surface regions of onemagnet plate facing and registering respectively with the poles and thetransitional surface regions of the other magnet plate, the oppositeface of said second magnet plate being directed to the atmosphere.
 6. Anelectroacoustic transducer as set forth in claim 1 in which the magnetplate is formed with a multiplicity of perforations, the walls of whichform exponential horns having their axes extending transversely of saidone face, each perforation in the magnet plate connecting the air spacebetween said diaphragm and said magnet plate with the atmosphere.
 7. Anelectroacoustic transducer as set forth in claim 1 in which saidpermanent magnet plate is magnetized predominantly at said one face andonly minimally at the opposite face.
 8. An electroacoustic transducer asset forth in claim 7 in which the magnet plate is made from amagnetically anisotropic material and thus having a direction ofpreferred magnetization, its actual magnetism being aligned with saiddirection of preferred magnetization, said actual magnetism beinggenerally parallel to said one face at the transitional surface region.9. An electroacoustic transducer as set forth in claim 1 in which saidone face of the magnet plate is circular and flat and the poles and thetransitional surface region are annular and concentric with said oneflat face.
 10. An electroacoustic transducer as set forth in claim 9 inwhich the vibratable diaphragm portion has circular corrugationsconcentric with said one flat face.
 11. An electroacoustic transducer asset forth in claim 9 which includes means secured to said magnet plateand holding the center portion of the diaphragm in stationary spacedrelation to said magnet plate.
 12. An electroacoustic transducer as setforth in claim 11 in which said holding means also is a fixed terminalmember connected to one end of the conductor.